1. Field of the Invention
The present invention relates to a method for charging a rechargeable battery at a constant voltage and a constant current, and a battery pack that charges a rechargeable battery based on this method.
2. Description of the Related Art
In the case where rechargeable batteries, typically, lithium ion batteries are charged, the batteries are charged at a constant current with the maximum of terminal voltage being limited to a setting voltage (constant current charging stage), while after the terminal voltage reaches the setting voltage the batteries are charged at a constant voltage (constant voltage charging stage). This charging method is referred to as constant-current and constant-voltage charging method, and is often used for rechargeable batteries. In some of this type of charging methods, before the charging operation is switched from the constant current charging stage to the constant voltage charging stage, the charging voltage or the charging current is gradually reduced. On the other hand, rechargeable batteries will be gradually deteriorated with the number of repeated charging/discharging cycles or if the rechargeable batteries are left standing (stored) for a long time after charged. If the deterioration degree of a rechargeable battery gets larger, the fully-charged capacity (FCC: Full Charge Capacity) or the safety may decrease. For this reason, it is important to grasp the degree of deterioration of rechargeable battery and to suppress the deterioration degree.
The life of a rechargeable battery can be determined by the degree of deterioration of rechargeable battery. The actual capacity (capacity equivalent to FCC) of a rechargeable battery is a capacity which can be substantially discharged from the rechargeable battery. The life and the actual capacity of a rechargeable battery will decrease with the number of repeated charging/discharging cycles or if the rechargeable battery is left standing for a long time. However, there is a tendency that the life and the actual capacity of a rechargeable battery will also decrease with the value(s) of setting voltage and/or setting current in charging operation. To address this, Japanese Patent Laid-Open Publication No. JP 2008-5644 A discloses a method that, in order to reduce (decrease) of actual capacity and increase the life of a rechargeable battery, decreases the setting voltage with the number of repeated charging/discharging cycles of the rechargeable battery.
Also, JP 2008-228492 A discloses a method which detects any of the internal resistance, actual capacity (learning capacity) and the number of charging/discharging cycles of a rechargeable battery to determine the deterioration state (degree of deterioration) of the rechargeable battery, and decreases the setting voltage depending on the determined deterioration state. In addition, JP 2002-236154 A discloses a method that corrects the decrease of the setting voltage by reducing the leaning capacity in accordance with the counted number of charging cycles and the elapsed storage time of the battery.
However, in the method disclosed in JP 2008-5644 A, since the setting voltage is decreased simply depending on the counted number of the charging/discharging cycles or the accumulated value of the charged/discharged capacities (capacities which are charged and/or discharged), even if the life of the rechargeable battery can be longer, the setting voltage is decreased without evaluation of the actual degree of deterioration. Also, in the method disclosed in JP 2008-228492, although a table or function is predetermined by modeling relationship between the deterioration state and the indices (e.g., internal resistance, actual capacity, and the number of charging/discharging cycles of the rechargeable battery) so that the deterioration state is determined based on the table or function, the decrease of setting voltage is not taken into consideration when the deterioration state is determined. Also, in the method disclosed in JP 2002-236154, this method is aimed at accurately correcting the learning capacity but is not directed to suppress the deterioration degree of the rechargeable battery.